postinst is in libwxgtk3.0-gtk3-dev 3.0.4+dfsg-3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | #!/bin/sh
set -e
update-alternatives --install /usr/bin/wx-config wx-config \
/usr/lib/x86_64-linux-gnu/wx/config/gtk3-unicode-3.0 \
$(( 30 * 10 + 7 ))
|